Three Agrobacterium tumefaciens mutants with chromosomal mutations that affect bacterial virulence were isolated by transposon mutagenesis. Two of the mutants were avirulent on all hosts tested. The third mutant, Ivr-211, was a host range mutant which was avirulent on BryophyUum diagremontiana, Nicotiana tabacum, N. Infection of susceptible dicotyledonous plants by Agrobacterium tumefaciens results in crown gall tumor formation. The mechanism by which the bacterium infects the plant is not fully understood. An early step in tumor formation involves the attachment of the bacterium to host plant cells (12). Once attached to the plant, the bacterium begins a process that results in the movement of tumor-inducing (Ti) plasmid DNA sequences from the bacterium into the plant host genome (2, 28). Depicker et al. have identified the DNA sequences transferred and termed them T-DNA (5). Without successful transfer of the T-DNA from the bacterium to the plant, tumor formation cannot occur. This paper examines the properties of three avirulent TnS insertion mutants of A. tumefaciens,. These TnS insertions are contained within the chromosome of the bacterium; thus, the insertions have no direct affect on the vir region of the Ti plasmid which is responsible for T-DNA transfer. The only previously identified chromosomal genes required for virulence are chvA and chvB (6), att (16), pscA (27) (also called exoC [1]), chvD (32), and chvE (9). Mutations in each of the first four genes listed result in the inability of the bacteria to attach to host cells and the consequent loss of bacterial virulence. Mutations in chvD result in reduced virulence on Bryophyllum diagremontiana (visible tumors were still formed) and reduced induction of virG by acetosyringone (32). Transposon insertions in chvE result in a * Corresponding author.

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